This application claims the priority of German patent document 198 56 499.6, filed Dec. 12, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process and apparatus for two-stage supercharging of process air for a fuel cell.
German Patent Document DE 43 18 818 A1 discloses a fuel cell system in which the process air is compressed by means of a compressor. For the recovery of energy, the fuel cell exhaust gas is fed to an expander which is coupled with the compressor by a shaft. The compressor and expander operate according to the displacement principle, and are constructed particularly as rotary machines. This system has the disadvantage that it is not suitable for fuel cell systems in which the fuel gas must first be generated by reforming methanol. In the case of fuel cell systems of this type, the exhaust gas temperature typically amounts to 350xc2x0 C. These temperatures are compatible with a displacement machine only to a limited extent.
U.S. Pat. No. 4,838,020 discloses a fuel cell system in which the process air is supercharged by means of an exhaust gas turbocharger. The process air to be fed to the fuel cell is compressed by means of a compressor. To recover energy from the fuel cell exhaust gas, the exhaust gas is guided to a turbine which is mechanically coupled with the compressor. In addition, a catalytic burner, which raises the energy content of the exhaust gas guided to the turbine, is arranged in the exhaust gas pipe.
In German patent document DE 197 55 116.5 (which is not prior art), a fuel cell system with a two-stage supercharging operation is described. An expansion machine is used in both stages. In this case, the turbine of the first stage is operated with the exhaust gas of the fuel cell. A catalytic burner, which raises the energy content of the exhaust gas guided to the turbine, is also arranged in the exhaust pipe. The turbine of the second stage is operated by the expansion of the highly pressurized gas from the high-pressure generating unit of the fuel cell system.
It is an object of the invention to provide a process and apparatus for two-stage supercharging of process air for a fuel cell which achieve a high pressure ratio, as well as a high efficiency.
This and other objects and advantages are achieved by the process and apparatus according to the invention, in which fuel cell process air is supercharged in two charging stages: by means of a displacement machine in one charging stage, and by means of a combined expansion and compression machine in the other charging stage. In this manner, the expansion of the fuel cell exhaust gas is used to compress the charge air, so that energy from the exhaust gas can be recovered.
In a multi-stage supercharging, a total pressure ratio ntotal is obtained as the product of the pressure ratios n1, n2 of the two compressor stages:
ntotal=n1*n2. 
A displacement machine is a compressor which takes in the conveying medium (gas or gas mixture) by the enlargement of working space (for example, cylinder), and displaces it by a subsequent reduction of the working space. As the result of this movement, the pressure of the conveying medium is increased. The main structural element of the displacement machine is a piston. Depending on the type of movement of this piston, a distinction is therefore made between reciprocating piston machines or rotary piston machines. Depending on whether the conveying medium is compressed inside or outside the machine, a differentiation is made between machines with and without an internal compression. Displacement machines are also called mechanical chargers because they compress the conveying medium by using mechanical energy.
In a combined expansion and compression machine, speed energy in a continuous gas flow is converted into pressure energy. It operates at high rotational speeds and high temperatures. According to the invention, particularly an exhaust gas turbocharger or a compression wave charger (also called Comprex(copyright) in literature) are used.
A high pressure ratio is achieved, without need of a high-pressure gas generating system as in German patent document DE 197 55 116.5.
Energy can be recovered from the exhaust gas of the fuel cell.
A high efficiency is achieved.
The invention can also be used at high exhaust gas temperatures, and is therefore particularly suitable for use in methanol fuel cell systems, in which the fuel gas is generated by the reforming of methanol.
In contrast to one-stage supercharging by means of an exhaust gas turbocharger, a higher flexibility is achieved with respect to the space distribution of the individual components.
The system can be implemented entirely without oil. This is of great significance in the case of a fuel cell system because the fuel cell is destroyed or limited in its efficiency by the oil.
The invention can be used for all types of fuel cell systems. It is particularly suitable for mobile applications, for example, in a motor vehicle or in railway systems.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.